METAMEKIC SEGMENTATION AND HOMOLOGY. 
245 
But then, it will be objected^ is homology to be reduced to 
a vague and. almost meaningless comparison ? Not at all ! 
Comparing a horse with a man, we may say that the trunk of 
the one is generally homologous with the trunk of the other; 
further, that the fore litnb of the horse is homologous with the 
fore limb of man ; and further still, that the one digit of the 
former is more specially homologous with the middle digit of 
the latter, and so on to the minutest detail — to the smallest 
blood-vessel, to a single nerve-fibre, even to a single cell. 
The homology may be perfectly definite, and pursued to the 
furthest conceivable limit. For it to be recognised it is only 
necessary that the structures compared should, at all events 
in theory, be traced to a common ancestral origin. 
The completeness of the homology depends on how far all 
the parts can be so traced back. The homology is impaired by 
the addition or loss of any parts. For instance, the pectoral 
girdle of the Teleostome fish is incompletely homologous with 
that of the Elasmobranch, since dermal bones of different 
origin have been added in the Teleostome to the primitive 
endoskeletal girdle, and it is incompletely homologous with 
the pectoral girdle of the mammal, because the latter has 
almost or entirely lost the dermal elements. Two organs are 
completely homologous when all their parts have been derived 
from corresponding parts in the common ancestor. 
Summary. 
In the Vertebrates, as in other animals, the organs and parts 
of two individuals are to be considered as homologous when 
they can be traced back to corresponding parts in a common 
ancestor, and not because they occur on the same segments. 
between these organs and segments of a certain nnnievical order ; and 
this relation is also more often found in animals composed of few 
segments than of many, and at the anterior than at the posterior end 
of the body (see further the articles on Arthropoda and Metamerism 
written by Sir E, R. Lankester in the ‘ Encycl. Brit.,’ eleventh edition, 
and reprinted in this journal, vol. 47). 
VOL. 59, PART 2. NEW^ SERIES. 
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